The 5101 Class were medium-sized tank engines used for suburban and local passenger services all over the Great Western Railway system. The class was an updated version, by Collett, of Churchward's 1903 3100/5100 Class.

The original 40 members of the 3100 class were renumbered 5100 and 5111 to 5149 in 1927. The first batches of 5101s filled in the numbers 5101 to 5110 and extended the class from 5150 to 5189. They were little changed from the Churchward locomotives as they then were, but had an increased axle loading of 17 long tons 12 cwt (39,400 lb or 17.9 t); the maximum permitted for the ‘Blue’ route availability. Bunkers were of the standard Collett design with greater coal capacity. The 5100 number series was exhausted in 1934, and further new locomotives were numbered from 4100. The last 20 were built after nationalisation.^[​1]